Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack. Due to these attributes, silicon nitride is useful in a variety of high wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. To date, substantial efforts have been made to prepare silicon nitride ceramics with high fracture toughness and flexure strength. Prior art silicon nitride ceramics, however, have only demonstrated fracture toughness values of about 6-8 MPa-m(1/2) and flexure strength values of about 800 MPa, at room temperature. The values for the flexure strength at 1200° C. are about 600-650 MPa, which limits the usefulness of prior art silicon nitrides in high temperature applications.
Thus, it would be very desirable to have a silicon nitride ceramic that exhibits high flexure strength and fracture toughness at both ambient temperature and at elevated temperatures. Moreover, it would be highly desirable to have a process which would be reproducible, inexpensive, and efficient for preparing silicon nitride ceramics that exhibit high flexure strength and fracture toughness.